Method of forming an electron emitting body



Aug. 5, 1958 J. N. DEMPSEY 5 9 METHOD OF FORMING AN ELECTRON EMITTING BODY Filed Oct. 21, 1955 PREPARE POROUS SPONGE METAL MATRIX IMPREGNATE MATRIX WITH ALKALINE EARTH FORMATE-ACETATE [e.g.= EQUI-MOLAR AQUEOUS SOLUTION OF aucoom Bo(CI-I COO)2] DECOMPOSE ALKALINE EARTH FORMATE-ACETATE TO OXIDE ACTIVATE CATHODE.

INVENTOR. JOHN N. DEMPSEY Bygw T/VEY I United States Patent METHOD OF FORMING AN ELECTRON EMITTING BODY John N. Dempsey, Edina, Minu., assignor to Minneapolis- Honeywell Regulator Company, Minneapolis, Minn.,

a corporation of Delaware Application October 21, 1953, Serial No. 387,515

4 Claims. (Cl. 117-423) pending application of Edward F. iRexer,'Serial N0.

387,537, filed October 21, 1953, assigned to the same 'assignee as the'present invention and entitled Electron Emitting Cathodes.

According to present day methods of producing elecbarium-strontium oxide, is sprayed. onto, brushed onto,

or mechanically compressed directly into the cathode base metal matrix. This oxide coating may be applied directly to the cathode body as such, or, it may be applied in-the tron emitting cathodes, the emitting material for example, 4

form of a carbonate and subsequently converted to the l oxide. Although these procedures have met with general approval, they leave room for considerable improvement, particularly since these cathodes are subject to rapid deterioration due to the tendency of the emissive coating to peel or flake away from the matrix after short periods of operation. This deficiency is even more pronounced when the unit is subjected to periods of rough handling or operation. Basically, there is insuflicient adherence between the base metal matrix and the emissive coating in 1 these present day devices, and the coating is thus subject to deterioration, particularly in instances of mechanical vibration or shock. Of course, when this emissive coating breaks away from the cathode body, there is no additional emissive material available, and the cathode as well as the device with which it is :being example, a vacuum tube is rendered useless.

The present invention provides an extremely rugged type of dispenser cathode in which the emissive coating tenaciously adheres or bonds itself to the'matrix body 'without need fora-n adhesive or bonding agent, and it is able to endure severe vibration and shock without peeling away or flaking ofi. In accordance with the present invention, the emissive coating forms as a continuous layer or continuurn within the porous matrix Which also s operated, for

has va continuous structure or continuum of metal throughout its extent. emission per unit area of cathode is achieved than is possible with conventional cathodes, thus, smaller cathodes maybe utilized for a given-rate of emission in accordance witlrthe present invention. Further, a low interface resistance between the base metal matrix and the emissive coating is obtained when the elements are made in accordance with the present 'invention.

The present invention provides an extremely simple and 'efiicient method of impregnation which may be accom- In addition, a higher. ratio of Isired in the base metalmatrix, since this Patented Aug. 5, .1858

plished with a single-step impregnation procedure, there: by eliminating possible sources of contamination. Therefore, it is an object of thepresent invention to produce an improved electron emitting cathode havingan electron emitting substance which is tenaciously bonded to the surface of the base metal matrix, thus producing a more rugged type of cathode structure.

It is afurther object of the present invention topmduce an electron emitting cathode which is able to endure mechanical shock and-vibration without losingthe-emis sive coating through flaking or peeling. v It is still a further object of the present invention'rto provide an electron emitting cathode whichhas a sub; stantially continuous supply of emittingmaterialavailable for thee'lectron emitting body. V An additional object of the present invention isit'o provide a dispenser-type electronemittingcathodewhich has a low interface resistance 'betwen the matrix body and the electron emitting-substance.

It is still a furtherobject of thepresent invention to provide an improved method'of producing electron emitting cathodes which employa single-step impregnation procedure, and produe'eavery tightly adhering alkaline earth metal oxide coatingon a base metal matrix.

Reference is made to the accompanying :drawingwhich shows in block diagram form a flow sheet of a suggested process in accordance with the present invention.

According 'topresent 'invention, a poro'us base metal matrix is prepared in accordance withconventional metallurgical procedures: For example, powdered nickel or any other suitable oxide cathode bas'e metal is thoroughly mixed with a volatilebinder, and a slurry made therefrom. The slurry, preferably being of pasty consistency, is then placed in a die and pressed with sufficient pressure to .cornpressthe slurry into the desired form. I prefer to use relatively low compressing pressures in order that the resulting matrix body ma be highly porous. The pressed bodies are thenf-sintered under a neutral or reducing atmosphere, for example, hydrogen, the binder preferably being volatilized;at' the sin-tering temperature so as to leave a porous base metal matrix. This matrix'is then cleanedto remov impurities therefrom and is-then ready for impregnation. When nickel is employed as the basefmetal, amatrix produced therefrom having a porosity of 20 to"$O is satisfactory. Porosity is defined as:

I P-(l 9100 wherein:

P=porosity in percent D. -weight of:base metal matrix per volume of matrix D =density of base metal I i .1?

As, a general rule, .as the powder size decreases, the number of pores increases, whichjis' desirable. Fonexam'ple, in a nickel base metal powder having a sizeranging from 37 to. 44 microns .a porosity .of 27.4%.has been found satisfactory, whereas a powdersize having a range of .under 37microns, pressing ,toa porosity of.2 9.2% was very satisfactory. It should be noted at thispoint, however, that a relatively high degree of porosity. is decreates a body wherein a substantial amount of the ,inner-iporeslareaccessibl-e from the outer surface of the body-.. Th' 5 upon the impregnation step which follows... i

In general, impregnation is carried out in a single step or operation, this step comprising introduction of a s0- lutionof aasoluble thermally dcomposablealkalineearth salt which is capable of thermal decomposition to the oxide thereof, .forexample, the -solubler .alkaliae earth metal salts of organic acids which are thermally decomis is based i posable to the oxide thereof. After impregnation, the base metal matrix is subjected to a temperature sufficient to decompose the alkaline earth metal salt to the oxide thereof. The volatile by-products of the decomposition are preferably removed as formed. Generally, the salts decompose to the carbonate which is then further decomposed to the oxide by heating to a sufficiently high temperature. After conversion to the oxide, the cathode is ready for activation. Activation may be accomplished by heating the oxide impregnated cathode body in the presence of a reducing agent to produce a quantity of free alkaline earth metal in the alkaline earth oxide. For example, activation is accomplished by heating the impregnated cathode body to a temperature of from 1000 C. to 1200 C. for a period of time sufticiently long to activate the body, but not for such a long period as will evaporate an appreciable amount of the activating material. For example, I have found that an activating period of from 2 to 3 minutes under these conditions is satisfactory. Where barium and strontium oxide is present in the cathode body, the activation reduces a portion of these oxides to the free metal, the barium being reduced in greater proportion than the strontium. Thus, there is created a continuous body or continuum of free alkaline earth metal and the oxide thereof. It is preferable to hold the element out of contact with gases such as air' in order to prevent the recombination of the oxide with carbon-dioxide or water, thus forming undesirable impurities in the cathode structure.

When a nickel base metal cathode matrix is employed, small amounts of aluminum may be present as a residual or added element. This aluminum acts as a reducing agent in the matrix and has been found satisfactory.

In accordance with the preferred modification of the present invention, a porous base metal matrix, prepared as above indicated is impregnated with a solution of barium and strontium acetates. A saturated solution of 50/50 mol ratio is preferred. This impregnation may be carried out by simply immersing the base metal matrix in the solution until the required amount of salt is contained in the matrix, or by placing the solution, for example by eye-dropper, onto the surface of the raw body. I prefer to utilize a sufficient amount of thermally decomposable salt in this step to provide for between 1 and 10% by weight of alkaline earth metal oxide in the porous base metal matrix. Following the impregnation, the matrix body is dried and then subjected to a heat treatment wherein the acetate salts are decomposed to the carbonates according to the following equations:

heat ll B8(CHsCOO)z B800: CHrC-CH:

Heating to a temperature of 1000 C. is generally sufficient for the first reaction to go to completion. The symmetrical ketone is preferably removed from the reaction chamber as quickly as possible, that is, on formation. After the acetates have been completely converted to the carbonates by purely thermal means, the temperature is increased to 1150 C. at which temperature the carbonates are decomposed into the respective oxides, the carbon dioxide by-product being removed as it is formed.

This impregnation may also be accomplished by a modified procedure. In this modified procedure, barium and strontium formate are impregnated into the porous matrix as in the case of the acetate salts. They are then decomposed according to the equations,

heat BB(GOOH): v ECHO BaCO; BaO C;

eat

out by utilizing the acetate-formate salt, according to the following reaction.

1 heat ZBMCOOHXOHaCOO) l heat 2Ba0 03 ZCHZCHO l heat Similar equations may be written for the corresponding strontium salts. This modified procedure has some advantage over the acetate method in that the acetaldehyde boils at a lower temperature than does acetone. It is to be noted, however, that closer control is necessary in this modification since the reaction is somewhat violent unless some care is taken to volatilize off the acetaldehyde relatively slowly.

Various other modifications are possible wherein soluble alkaline earth metal salts of higher organic acids are utilized, for example, the propionate and butyrates. In this case, if a single salt is used, the by-pr-oduct formed is the symmetrical ketone, a mixture of these higher salts yields an unsymmetrical ketone; and in case any of these salts are utilized in conjunction with the formate, the by-product formed is the corresponding aldehyde. After decomposition of the carbonate to the oxide in each of these cases, the cathode body is activated as previously described.

If desired, the cathode matrix may be first mounted in its assembly, regardless of its complexity, and thereafter impregnated in accordance with the materials and methods set forth above with the additional utilization of an eye-dropper to dispatch the solutions onto the base metal matrix where desired. Such a procedure is readily adaptable to production methods.

Many details of process and procedure may be varied without departing from the principles of this invention. It is therefore not my purpose to limit the scope of this invention other than necessitated by the scope of the appended claims.

I claim as my invention:

1. The method of forming an electron emitting body which includes the steps of impregnating a porous metal sponge matrix with an aqueous solution of an alkaline earth metal formate and an alkaline earth metal acetate, said formate and said acetate being employed in substantially equi-molar proportions, thermally decomposing said salts to the oxide form'in-situ and then activating said oxide impregnated matrix to create electron emitting characteristics.

2. The method of producing an electron emitting cathode which includes the steps of impregnating a porous base metal pellet with an aqueous solution of bariumstrontium acetate and barium-strontium formate said acetate and formate being in substantially equi-molar proportions, thermally decomposing said salts to the oxide form, and then activating said oxide impregnated matrix to create electron emitting characteristics.

3. The method as defined in claim 2 being further characterized in that said porous metal pellet consists essentially of nickel.

4. The method of producing an electron emitting cathode which includes the steps of impregnating a nickel pellet with an aqueous solution of barium-strontium acetate and barium-strontium formate, said acetate and formate being in substantially equi-molar portions, thermally decomposing said salts to the oxide, and then activating said oxide impregnating matrix 'by heating to a temperaing agent.

References Cited in the file of this patent UNITED STATES PATENTS Harris Apr. 7, 1925 Rufliey Jan. 10, 1933 6 5 McCulloch Dec. 12, 1933 Pearcy Nov. 20, 1934' Ramsey et a1 June 29, 1937 Kurtz Nov. 13, 1945 Rouse D60. 21, 1948 Macksoud Oct. 10, 1950 Coppola 'et a1. Nov. 6, 1956 

1. THE METHOD OF FORMING AN ELECTRON EMITTING BODY WHICH INCLUDES THE STEPS OF IMPREGNATING A POROUS METAL SPONGE MATRIX WITH AN AQUEOUS SOLUITION OF AN ALKALINE EARTH METAL FORMATE AND AN ALKALINE EARTH METAL ACETATE, SAID FORMATE AND SAID ACETATE BEING EMPLOYED IN SUBSTANTIALLY EQUI-MOLAR PROPORATIONS, THERMALLY DECOMPOSING SAID SALTS TO THE OXIDE FORM IN-SITU AND THEN ACTIVATING SAID OXIDE IMPREGNATED MATRIX TO CREATE ELECTRON EMITTING CHARACTERISTICS. 